This invention relates to digital television and, more particularly, to methods and apparatus for filtering digital television signals to remove multipath and other undesirable effects upon a digital television signal as the signal propagates through a channel.
Digital television is an emerging technology that is the subject of much research both in the United States and Japan. Because of the potential advantages of digital television and the many technical problems associated therewith, research into improved systems and methods for transmitting and receiving digital television signals is increasing.
One of the most important prevalent problems associated with digital television signals is the problem of multipath effects. The term multipath, as used herein, refers to the propagation of electromagnetic waves along various paths from the digital television transmitter to the digital television receiver. Multipath effects may arise from fixed structures, such as building walls, acting as reflectors in the transmission channel. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath conditions. These structures can cause transmission of the television signal to occur along more than one path from the transmitter to the receiver. As a result, the same signal may be received more than once, and at different times by a single, or multiple, receivers. The result of multipath effects in analog television is to create "ghosts" in the displayed television image. In digital television, the effects of multipath can include moderate to severe degradation in the displayed TV picture and sound.
Various methods and systems have been designed to address the problem of multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, "Design of a Ghost Canceling
Reference Signal for Television Systems in North America," Proceedings of Canadian Conference on Electrical and Computer Engineering, Vancouver, BC, Canada, Sep. 14-17, 1993, pp. 660-663.
The statistics of multipath ghosts have been studied and compiled by, among others, the BTA (Japan's Broadcasting Technology Association). The BTA, and other concerns, designed a "ghost canceling reference" (GCR) transmitted signal to mitigate these multipath effects. The BTA GCR was found to be less than satisfactory in some cases. While homes with outdoor antennas displayed non-varying (stationary) ghosting conditions which could be largely corrected, those homes with indoor antennas experienced changing (dynamic) ghosts. These ghosting conditions were more prevalent where people were moving about the room or other moving objects were in the signal path. The BTA ghost canceller generally was not able to adequately compensate for these conditions. Therefore, a need remains for a system and method for filtering out, or removing, multipath components from digital television signals, and especially for systems and methods for filtering multipath components from a digital television signal when the multipath component arises from moving objects and dynamic conditions in a transmission channel.